This invention relates to analysis of liquid samples (e.g. water) to determine the presence of a target analyte. More particularly, this invention relates to analysis of liquid samples to determine the amount of a volatile component produced or liberated from the liquid sample.
It is often necessary or desirable to analyze a liquid sample (such as water) to determine the presence of various contaminants or components therein. For example, it is necessary to analyze water to determine the amount of carbonaceous material which may be present. This is often referred to as a xe2x80x9ctotal carbonxe2x80x9d test. In some applications, the total carbon test can be further differentiated to xe2x80x9corganic carbonxe2x80x9d and xe2x80x9cinorganic carbonxe2x80x9d. Examples of organic carbon compounds would include carbon associated with amino acids or proteins. An example of inorganic carbon would be carbonates in water.
One conventional test method for determining the presence of carbon-containing material in water involves combustion of the sample to form carbon dioxide (CO2) and determination of carbon dioxide by a nondispersive infrared analyzer. The disadvantages of such method include the high temperature, oxygen and catalysts needed to breakdown the carbon compounds to gaseous carbon dioxide and the necessity of purging, drying and carrier gas-transfer of the carbon dioxide to the analyzer. Instrumentation of this type is generally expensive and dedicated just for carbon measurements.
The principle of microdiffusion analysis is described by Conway, Edward J., Microdiffusion Analysis and Volumetric Error, Crosby Lockwood and Son Ltd., London, 5th Ed., 1962. It involves simple gaseous diffusion of a volatile substance from an outer chamber, where it exerts a certain tension, into an inner chamber, where the tension is zero on the surface of the absorbing fluid. At room temperature and at normal atmospheric pressure, the rate of diffusion of a volatile component is determined by its vapor pressure.
Conway determined that the diffusion process could rapidly reach completion using very small volumes of sample and reagents (typically less than 2 mL); thus, it was referred to as xe2x80x9cmicrodiffusionxe2x80x9d. A major challenge to microdiffusion analysis was the precise volumetric measurements required for accurate results. Several micro-pipets and micro-buret designs were utilized by Conway for microdiffusion analysis.
Conway designed a special diffusion apparatus (a xe2x80x9cunitxe2x80x9d) which in one form resembled a covered petri dish with concentric compartments. For example, in the microdiffusion procedure for ammonia, 1 mL of standard base solution is pipetted to the outer chamber of the unit. Then 1 mL of standard acid is pipetted to the inner chamber. A volume of sample (typically less than 1 mL) which contains ammonia, is added to the base reagent in the outer chamber. The unit lid is smeared with a fixative and then sealed, after which the unit is rotated in a circular motion to mix the sample with base solution. For ammonia, the unit is left to sit at room temperature or is continuously swirled in a rotating motion using an oscillating table. After a suitable time for absorption, the lid is removed and the contents of the inner chamber is removed for analysis, either by titration or calorimetric measurement.
Conway mainly employed isothermal diffusion, usually at room temperature. The rate of diffusion was controlled mainly by the volatile component""s vapor pressure and small sample volume.
Conway""s method for microdiffusion analyses does not allow much flexibility. Careful metering of reagents and sample volumes are required. Complete diffusion of the volatile component did not often occur using such method.
Kirk, P. L., Anal. Chem., 22, 611 (1950) described a distillation-diffusion unit which utilized differential temperature for distillation. In the Kirk unit, the inner chamber, containing the absorbing media, is maintained at a lower temperature than the outer chamber, which contained the volatile component. The differential in temperature would aid the separation of constituents (such as volatile alcohols) for which there is no good chemical absorbent. However, in Kirk""s design, it was necessary to prevent pressure developed during heating, which could blow the seal open. Thus, Kirk""s unit incorporated an evacuation design to prevent pressure build-up in the test unit.
Other microdiffusion units are described by Koga et al., Shika Gakuho, 90(7), 979-82 (1990); Hinoide et al., Journal of Dental Health, 40, 254-55 (1990); Heyer, East German Patent DD 213065 (1984); and Grosse, Russian Patent SU 1623747 (1991). The Koga, et al. and the Hinoide, et al. units are of similar design to a Conway unit and are made of TEFLON(copyright) and include a screw-cap closure. Like the Conway unit, microdiffusion is optimized by a horizontal orientation to maximize the surface area of the exposed sample and by limiting the sample size. The TEFLON unit probably could not be used at temperatures above the reported 90xc2x0 C., and it must be disassembled to allow access to the trapping solution in the inner chamber for the analysis.
The apparatus described by Heyer was designed for separation of volatile components of mixtures. The mixture is placed in a cylinder which can be heated. An absorption chamber, which contains a sealing liquid, surrounds the top of the cylinder. A bell jar is inverted over the absorption chamber which serves to trap and condense the volatile component into the sealing liquid. Heyer""s apparatus is principally based on heat convection as an aid to separation of components. Isothermal diffusion would not work with this design. The seal is accomplished using a liquid trap which absorbs the volatile component. The heat and pressure employed with this design must be limited to quantitatively trap the volatile gas. The absorbed material must be transferred out of the absorption chamber for measurement.
Grosse describes a cylinder cup with a spout and concave bottom. The absorbing medium is placed into this cup. A smaller cup is placed on the concave support of the larger cup and contains the sample and releasing reagent(s). A dome cover is placed over the assembly which integrates airspace between the two cups. The stated advantage of his design is that the assembly can be heated to accelerate the diffusion process. Because of the seal design, however, there is a practical limitation on the amount of heat and pressure that can be used with such unit. For example, the weight of the dome lid and its contact with the larger cup floor would be factors for maintaining adequate sealing. Like Conway""s unit, the Grosse unit is based on using a small sample volume and large surface areas to maximize the diffusion process. This necessitates the horizontal orientation of the device. It is also necessary to remove the trapped analyte from the absorption cup in order to analyze it.
The main objection to the use of micro-diffusion analyses has been the comparative slowness of the diffusion and the strict adherence to controlling factors which affect precision and accuracy of the method. Depending upon the particular unit used, isothermal diffusion could require up to 12 hours for complete transfer of the volatile component. Certain materials have a low vapor pressure and are difficult to diffuse isothermally.
There has not heretofore been provided a relatively simple and reliable controlled diffusion test method or procedure which has the features and advantages provided by the present invention.
In accordance with the present invention, there is provided an improved controlled diffusion test method for determining the presence and amount of a target analyte in a sample (which may be a liquid solution or a solid substance) which would volatize the sample analyte of interest. The test method involves the use of two separate tubular vessels of different diameters. The sample being tested is placed into the larger diameter tube or vessel, and an absorbing medium (which is a liquid solution) which serves as an indicator is placed in the smaller tube or vessel. Any necessary digestion reagents or reagents needed to volatize the analyte are added to the sample in the larger tube.
Then the smaller tube is placed inside of the larger tube, and a cap or closure member encloses the top end of the large tube and provides a gas-tight seal thereon. The assembly is then heated to an elevated temperature, whereby a volatile component (a target analyte) is produced or liberated from the test sample and enters into the open top of the smaller diameter tube where it is absorbed by the indicator.
Typically, the indicator changes color as it absorbs the target analyte. The extent of the color change may be easily measured optically to provide a quantitative determination of the amount of target analyte absorbed. For example, the extent of the color change can be measured with a spectrophotometer, a calorimeter, or a filter photometer.
The optical measurement can even be performed by passing a transverse light beam directly through both of the tubes without disassembly of the system when both the test sample and the indicator are liquids. This technique provides simplicity and avoids the need to separate the tubes or to remove any of the indicator from the inner tube, thereby avoiding possible spillage or contamination of the indicator.
The technique of the invention is useful for measuring a wide variety of target analytes, including for example, volatile amines, ammonia, antimony, arsenic, azide, cyanide, formaldehyde, halogens, ketones, tertiary nitrogen compounds, mercury, nitrates, nitrites, organic substances, phenols, sulfides, inorganic or organic halogen compounds, total nitrogen, oxygen demand, carbon, volatile organic carbon, total inorganic carbon, total organic carbon, particulate and non-purgable organic carbon, volatile acids, volatile alcohols, and other volatile organic compounds.
The techniques of the present invention provide a means for completely digesting hard-to-digest compounds, efficiently diffuse the resulting volatile component into a trapping media and measure the component concentration directly. The tube assemblies can be disposable, thereby avoiding the potential for contamination through re-use. The compactness of the system is also an advantage in laboratories where space is at a premium.
The invention does not require dedicated instrumentation such as is required with the conventional combustion-nondispersive infrared method. The invention uses spectrophotometers, calorimeters or filter photometers which are commonly available at analytical laboratories. Since the instrumentation and skill level to perform the test of the invention is minimal, the cost per test is much less than the conventional test methods.